1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting downlink control information.
2. Related Art
Recently, to maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases.
The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. The weight can be expressed by a weight vector or a weight matrix. The weight vector is referred to as a preceding vector. The weight matrix is referred to as a preceding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing.
The single-user spatial multiplexing is also referred to as single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as spatial division multiple access (SDMA) or multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt and the number of Rx antennas is Nr, the number of independent channels is Ni where Ni≦min{Nt, Mr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigenvalues of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed.
Control information for data transmission is transmitted to a user equipment (UE) through a downlink control channel. Downlink control information includes various types of information required for data transmission and reception. The UE may transmit data by receiving control information through the downlink control channel. The downlink control information is configured with several formats according to data to be transmitted. In the MIMO system, the downlink control information further includes preceding information. The preceding information may be unnecessary according to a data transmission mechanism of the MIMO system. For example, when using the SU-MIMO, the UE selects a specific frequency band from a wideband and transmits a preceding matrix indicator (PMI) for the selected frequency band, and when using the MU-MIMO, the UE transmits only a PMI for the wideband. A confirmation message is transmitted for the PMI transmitted by the UE, but is unnecessary information when using the MU-MIMO.
Accordingly, there is a need for a method for preventing unnecessary information from being included in downlink control information according to a predetermined format and for effectively transmitting the downlink control information.